1. Field of the Invention
This invention relates to a vacuum optical system equipped with vacuum chambers for incorporating an optical system used in a vacuum, and in particular, to an optical system using soft X rays.
2. Description of the Related Art
The development in recent years of the source of soft X rays, such as radiation, has promoted the research of soft X-ray optics. For semiconductor exposure devices, line widths being transferred become finer because of the microstructure of their integrated circuits, and the wavelengths of light used in exposure extend from the ultraviolet region to the X-ray region. For microscopes, attention is devoted to observations in the so-called "water window" region of wavelengths from 22 to 44 .ANG. which is generally reputed to be suitable for microscopy of biological specimens. As for analytical devices, a great number of absorption edges of elements exist in the X-ray region, thereby allowing the application to elementary analysis to be attempted. Further, the research and development of X-ray optical elements are promoted, and various X-ray microscopes are proposed which use the X-ray optical elements as objective lenses.
FIG. 1 shows an imaging X-ray microscope making use of a grazing-incidence Wolter optical system. In this diagram, reference numeral 1 represents a radiation source; 2 a condenser mirror; 3 a sample; and 4 a Wolter optical system. The X-ray microscope of the type has the feature of enabling radiation ranging in short wavelength to several angstroms to be imaged. The Wolter optical system 4, which is capable of imaging white radiation, can be used as an X-ray collecting lens for a probe where secondary electrons discharged from the sample 3 are observed.
FIG. 2 illustrates a scanning optical microscope making use of zone plates. This microscope is designed so that white X rays emitted from a synchrotron radiation source 10 are monochromatized with two zone plates, a condenser zone plate 11 and a micro-zone plate 12, and focused on a sample 13, and so that a transmitted microscope image of the sample 13 is photographed and displayed on a CRT monitor 16. The sample 13 is moved in the direction of a double-pointed arrow A by a driving motor 14 and a scanning mechanism 15, while the micro-zone plate 12 is moved in the direction of a double-pointed arrow B by a voice coil 17. The sample 13 is thus scanned two-dimensionally. The X-ray microscope of the type has the feature that arbitrary monochromatic X rays can be focused by shifting the position of each zone plate.
FIG. 3 shows a scanning X-ray microscope utilizing a laser plasma radiation source and a Schwarzschild optical system. This microscope is constructed so that laser radiation emitted from a laser radiation source 20 is focused on a target 21 to produce X rays, which are imaged, through a pinhole 22 and a falter 23, on a sample 25 in a Schwarzschild optical system 24, and the X rays transmitted through the sample 25 are detected by a detector 26. Reference numeral 27 denotes a scanning stage for scanning the sample 25 two-dimensionally. The X-ray microscope of the type employs a compact, high-luminance laser plasma radiation source, and thus is expected to be used as a laboratory-use microscope. The Schwarzschild optical system 24, in which the surfaces of two spherical mirrors are coated with soft X-ray multilayer films, is capable of imaging soft X-rays of particular wavelength with a spatial resolving power as high as about 50 nm.
Of the microscopes mentioned above, the scanning X-ray microscope has the feature that the transmitted microscope image of the sample can be obtained with a high resolving power defined by the optical system, irrespective of the spatial resolving power of the detector. The detection of photoelectrons and scattered X rays in addition to transmitted X rays makes it possible to secure the microscope images of various data (for example, data of particular protein contained in the sample) except for the transmitted X-ray image of the sample. Further, there is the advantage that the use of the laser plasma radiation source leads to a compact system excellent in function.
Since soft X rays are considerably absorbed into the air, it is necessary to incorporate an optical system, such as the scanning X-ray microscope, in a vacuum as shown in FIG. 3. For means of supporting the optical system, there is a manipulator attached to a vacuum flange and a stage mounted directly to a vacuum chamber, with which optical alignment is performed.
The vacuum optical system of the prior art has the problems that (a) the stages which are the mounting bases of the optical system, as shown in FIG. 4, are fixed integral with or directly to the vacuum chamber or the vacuum flange, and if evacuation is performed after the alignment in the air, as shown in FIG. 5, deformation of the vacuum chamber due to the atmospheric pressure will cause misalignment, (b) the drive of the stage for a moving mechanism lying in the vacuum chamber in a vacuum state needs a vacuum motor which provides a heat discharge and a piezoelectric (PZT) element using high voltage, and thus the entire device becomes oversized, (c) the observation with soft X rays needs a long time to attain a high vacuum necessary for a high voltage detector, such as a photomultiplier tube, and (d) a rapid air flow caused by the evacuation may break a thin film for enclosing an object for observation, an ultrathin film filter used to remove unwanted light in the soft X-ray observation, and a thin film window for taking out soft X rays from the vacuum chamber. Hence, the prior art vacuum optical system is not necessarily used favorably.
Further, where the laser plasma radiation source is used as the radiation source of the scanning X-ray microscope, there is the problem that scattered particles discharged from the source may break a filter for removing ultraviolet rays, or may contaminate the mirror surfaces of the Schwarzschild optical system. Moreover, in the scanning X-ray microscope in which the pinhole is placed in front of the source, as mentioned above, and the image of the pinhole is formed by an X-ray optical element to produce a microbeam, the problem has been encountered that the intensity of X rays emitted from the target varies because of flickering of the laser beam, and the intensity of the microbeam also varies, with the result that the quality of the microscope image deteriorates.
Although the above problems arise from the fact that soft X rays must be treated in a vacuum, the same problems hold for any optical system which must be treated in a vacuum. For the optical system which must be treated in a vacuum, an example is shown in FIG. 6, in which a Fabry-Perot interferometer is provided in a vacuum chamber for the absolute measurement of wavelength of a stabilized laser (C. F. Bruce and R. M. Duffy, "Scanning Fabry-Perot interferometer for precision measurement", Rev. Sci. Instrum., Vol. 46, No. 4, pp. 379-382, 1975). In this case, a measurement is made by leaking a small amount of gas to a chamber held in a vacuum state, and the same problems arise from the deformation of the vacuum chamber caused by a change of the difference between the internal and external pressures of the vacuum chamber.
Further, there is another problem, although it is not present in the prior art, that under the presence of air, an error in measurement is produced by the disturbance of air. For example, a measuring device using a Mach-Zehnder interferometer, such as that shown in FIG. 7, is adapted to measure the path difference of split light for photomerry. Hence, this device raises the difficulty that where the measurement Is made with the tolerance of a hundredth of the wavelength of the light, a portion A surrounded by a broken line shown in FIG. 7, for splitting and recombining the optical path, has a significant effect on the result of the measurement in accordance with the disturbance and density of air.